1. Field of the Invention
The present disclosure relates to a density correction technique in an image forming apparatus such as a color printer or a color copier.
2. Description of the Related Art
There are image forming apparatuses that have multiple image forming units and form multi-color images by forming various colors of images with the image forming units and then transferring the images to an intermediate transfer member or a recording material in an overlapped manner. So-called color shift (misregistration), in which relative positions do not match between the images formed by the image forming units, occurs in these types of image forming apparatuses. Misregistration occurs due to error in the attachment of the members constituting the image forming units and due to the relative positions of these members changing due to variation in environmental conditions such as the temperature. Misregistration also occurs due to uneven rotation of rotationally-driven members, variation in the rotation speed, and the like. Also, the color balance (so-called color tone) changes due to variation in the image density for the various colors caused by conditions such as the usage environment and the number of printed sheets.
For this reason, Japanese Patent Laid-Open Nos. 01-167769, 11-143171, and 2001-166553 disclose configurations in which misregistration correction and density correction are performed by forming a misregistration detection pattern for detecting the amount of misregistration and a density detection pattern for detecting the difference between the desired density and the density that is actually formed. In Japanese Patent Laid-Open Nos. 01-167769 and 11-143171, misregistration correction and density correction are independently executed in separate processes (sequences). On the other hand, Japanese Patent Laid-Open No. 2001-166553 discloses a configuration in which the misregistration detection pattern and the density detection pattern are formed in the same sequence, and the two types of correction control are performed together in the same sequence, in order to shorten the downtime caused by these processes.
The misregistration amount caused by uneven rotation of the rotating members, variation in the rotation speed, and the like changes according to change in the speed. For this reason, in order to precisely determine the misregistration amount that has occurred, it is effective to form the misregistration detection pattern multiple times at different positions on the image carriers or the like. If misregistration correction and density correction are performed in the same sequence in order to shorten the downtime, the misregistration detection pattern and the density detection pattern need to be formed within one full rotation of the intermediate transfer member serving as the image carrier, for example. In other words, the length of the misregistration detection pattern needs to be less than or equal to the result of subtracting the length required for forming the density detection pattern from the length of one full rotation of the image carrier.
Accordingly, a short length is preferable for the density detection pattern formation range in the movement direction of the surface of the image carrier. Note that this also applies to the case where misregistration correction and density correction are not executed in the same sequence. This is because shortening the density detection pattern formation range in the movement direction of the surface of the image carrier shortens the time required for density correction control and shortens the downtime.